Stephen Hales and the cohesion theory

Abstract

In science, theories can be born more than once, as the mendelian laws have shown. In this context, the cohesion theory of water movement in plants has been variously ascribed to Josef Bohm1xCapillaritat und Saftsteigen. Bohm, J. Ber. Dtsch. Bot. Ges. 1893; 11: 203–212See all References1, Henry H. Dixon and John Joly2xOn the ascent of sap. Dixon, H.H. and Joly, J. Ann. Bot. 1894; 8: 468–470See all References2, and Eugen Askenasy3xUber das Saftsteigen. Askenasy, E. Verhandl. d. Heidelb. Naturhist.-Med. Vereins N. Serie. 1895; V: 325–345See all References3. However, all of the elements of this theory were first described in 1727 by the English clergyman Stephen Hales in his book Vegetable Statics4xSee all References4. Unfortunately, Hales' ideas were not understood at the time, so his findings failed to influence the debate on water transport in plants in the 19th century.Hales' insight appears to have been influenced by his relations with Isaac Newton. Although Newton left Cambridge in 1696 when Hales entered, they met in 1718 when Hales was elected a fellow of the Royal Society. As chairman of the society, Newton gave Hales' book his imprimatur.The most relevant section of Vegetable Statics is exp.33. After citing Newton's Optics (2nd edn, 1717; query 31, in which mercury is lifted 60–70 inches in a barometer tube by cohesion compared with a water pillar of over 60 feet) Hales wrote, ‘And by the same principle it is, that we see, in the preceding experiments, plants imbibe moisture so vigorously up their fine capillary vessels; which moisture, as it is carried off in perspiration, (by the action of warmth) thereby gives the sap-vessels liberty to be almost continually attracting of fresh supplies; which they could not do, if they were full saturate with moisture: for without perspiration the sap must necessary stagnate, notwithstanding the sap-vessels are so curiously adapted by their exceeding fineness, to raise a sap to great heights, in a reciprocal proportion to their very minute diameters.' Hales' discussion of water conduction in plants is based on sound experiments, such as his measurements of tensions in transpiring branches. He noted that tensions of up to 12 inches of mercury are not the full tension as air is sucked out of a branch simultaneously. He also produced an early dendrometer, and measured imbibition forces in peas.As with so many original arguments, Hales' sounds superficial on first reading, although there can be no doubt of the mechanism involved. Hales also failed to name his theory. Thus, four editions and translations of his book were insufficient to connect the name of this versatile clergyman with the cohesion theory.